New concrete will not stick to loose, crumbling concrete. You must create a sound surface.
| Mistake | Why It Fails | |--------|---------------| | Patching over loose concrete | The new patch will crack as the old layer moves or crumbles beneath it. | | Using regular mortar or sand mix | These lack polymers for adhesion; they’ll pop off in the first freeze-thaw cycle. | | Skipping the bonding agent | The patch will shrink slightly and separate from the old concrete. | | Making the patch too thick (over 2”) | Vinyl patchers are for thin repairs. For deep holes, build up in ½” layers. | | Curing too fast | Rapid drying causes shrinkage cracks and weak bond. | how to repair crumbling concrete window sill